Atomic Structure

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• Chapter 4
• Atomic Structure

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4.1 Defining the Atom

• An Atom is the smallest particle of an element
  that retains its identity in a chemical reaction.
  
• The Greek Philosopher
• Democritus, was among the
• First to suggest the existence
• Of atoms.

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• Democritus believed that atoms were indivisible
  and indestructible.
• Although Democrituss ideas agreed with later
  scientific theory, they did not explain chemical
  behavior.
• They also lacked experimental support.

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Daltons Atomic Theory

1. All elements are composed of tiny indivisible
   particles called atoms.
2. Atoms of the same element are identical. The
   atoms of any one element are different from those
   of any other element.
3. Atoms of different elements can physically mix
   together or can chemically combine in simple
   whole-number ratios to form compounds.
4. Chemical reactions occur when atoms are
   separated, joined, or rearranged.

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4.2 Structure of the Nuclear Atom

• Much of Daltons atomic theory is accepted today.
  One important change however, is that atoms are
  now known to be divisible.
• Three kinds of subatomic particles are electrons,
  protons, and neutrons.

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Electrons

• In 1897, English physicist J.J. Thomson
  discovered the Electron.
• Electrons are negatively charged subatomic
  particles.
• Thomson called these particles corpuscles later
  they were named electrons.

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Robert A. Millikan (1868-1953)

• Carried out experiments to find the quantity of
  charge carried by an electron.
• An electron carries exactly 1 negative charge and
  its mass is 1/1840 the mass of a hydrogen atom.

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• You need to understand.
• Atoms have NO CHARGE!!
• They are electrically neutral.

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• Next..
• Electric charges are carried by particles of
  matter.

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Third

• There are NO fractions of charges, only whole
  number multiples!!!

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And Finally.

• When a negative particle combines with a positive
  particle an electrically neutral particle is
  formed.
• 1 -1 0

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Eugene Goldstein

• Observed a cathode-ray tube and found rays
  traveling in the direction opposite to that of
  the cathode rays.
• He called these rays canal rays and concluded
  that they were composed of positive particles.
• These positive particles are called Protons.

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James Chadwick

• In 1932, an English Physicist confirmed the
  existence of yet another subatomic particle The
  Neutron.
• A neutron is a subatomic particle with no charge
  but with a mass nearly equal to that of a proton.

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Plum Pudding Model

• In Thomsons atomic model, known as the plum
  pudding model, electrons were stuck into a lump
  of positive charge, similar to raisins stuck in
  dough.

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• This model of the atom turned out to be short
  lived, however, due to the groundbreaking work of
  Ernest Rutherford (1871-1937), a former student
  of Thomson.

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Rutherfords Gold Foil Experiment
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Rutherford Atomic Model

• He proposed that the atom is mostly empty space,
  thus explaining the lack of deflection of most of
  the alpha particles.
• He concluded that all the positive charge and
  almost all the mass are concentrated in a small
  region.
• He called this region the Nucleus.
• The nucleus is the tiny central core of an atom
  and is composed of protons and neutrons.

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Key Concept

• In the nuclear atom, the protons and neutrons are
  located in the nucleus. The electrons are
  distributed around the nucleus and occupy almost
  all the volume of the atom.

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4.3 Distinguishing Among Atoms

• Elements are different because they contain
  different numbers of protons.

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Atomic Number

• The atomic number of an element is the number of
  protons in the nucleus of an atom of that element.

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Mass Number

• The total number of protons and neutrons in an
  atom is called the mass number.
• Number of neutrons mass number atomic number
• The number of neutrons in an atom is the
  difference between the mass number and atomic
  number.

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• How do you calculate mass number?

• If you know the atomic number and mass number of
  an atom of any element, you can determine the
  atoms composition.

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Isotopes

• Isotopes are atoms that have the same number of
  protons but different numbers of neutrons.
• Because isotopes of an element have different
  numbers of neutrons, they have different mass
  numbers.

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Isotopes Cont

• Atomic mass unit is defined as one twelfth of the
  mass of a carbon-12 atom.
• In nature most elements occur as a mixture of two
  or more isotopes.
• Isotopes of an element has a fixed mass and a
  natural percent abundance.

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Relative Abundance

• The Atomic Mass of an element is a weighted
  average mass of the atoms in a naturally
  occurring sample of the element.
• A weighted average mass reflects both the mass
  and the relative abundance of the isotopes as
  they occur in nature.

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Determine Atomic Mass

• stable isotopes
• Mass of each isotope
• Natural abundance of each isotope
• Atomic Mass Mass of Isotope x natural abundance
  add them together
• Look on page 117 for practice problems!!

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The Periodic Table A Preview

• Each horizontal row of the periodic table is
  called a PERIOD.

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Periodic Table Cont.

• Each vertical column of the periodic table is
  called a group.
• Elements within a group have similar chemical and
  physical properties.

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Lets Review!!!

• Atoms are the smallest particle of an element
  that retains its identity in a chemical reaction.
• Democritus believed that atoms were indivisible
  and indestructible.
• Daltons atomic theory states that
• All elements are composed of atoms
• Atoms of different elements combine to form
  compounds
• Atoms of the same element are identical
• Chemical reactions occur when atoms are
  separated, joined, or rearranged.

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Review Cont.

• There are 3 subatomic particles
• Proton
• Identifies the atom
• Positive charge
• Electron
• Determines chemical reactions
• Negative Charge
• Neutron
• Determines Isotope
• Neutral Charge

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Review Cont

• Thomson atomic model, known as the plum pudding
  model, electrons were stuck into a lump of
  positive charge.
• Rutherfords Gold Foil Experiment He proposed
  that atoms are mostly empty space.
• The nucleus is the tiny central core of an atom
  made of protons and neutrons. Electrons surround
  the nucleus.
• Atomic number is the number of protons.
• Mass number is the of protons and neutrons.
• Isotopes are atoms that have the same number of
  protons but a different number of neutrons.

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Review Cont

• Atomic mass unit is defined as one twelfth of the
  mass of a carbon-12 atom.
• The atomic mass of an element is the weighted
  average mass of the atoms in a naturally
  occurring sample of the element.
• To calculate the atomic mass of an element,
  multiply the mass by its natural abundance and
  then add the products.

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Review Cont

• Periodic Table allows you to easily compare the
  properties of one element to another element.
• Groups go down!!
• Periods all across!!
• Elements within a group has similar physical and
  chemical properties.